Modern digital processing hardware is becoming more and more sophisticated in its incorporation of automated diagnostics. A digital processing system with automated diagnostics can be capable of several different functions including fault detection and fault isolation. Faults or failures within these digital processing systems typically manifest themselves as short circuits between a particular voltage node or net and either ground or a voltage supply. These faults are generally termed stuck-at-1, stuck-at-0, or intermittent at 1 or 0. Once a fault is isolated by the diagnostics, the system can be quickly repaired thereby reducing down-time and increasing productivity. This makes automated diagnostics a very valuable feature in modern digital processing.
Examples of systems incorporating automated diagnostics in complex digital processing systems are documented in N. N. Tendolkar and R. L. Swann, "Automated Diagnostic Methodology for the IBM 3081 Processor Complex," IBM J. Res. Develop., Vol. 26, No. 1, January 1982 at 78-88, and in D. C. Bossen and M. Y. Hsiao, "Model for Transient and Permanent Error-Detection and Fault-Isolation Coverage," IBM J. Res. Develop., Vol. 26, No. 1, January 1982 at 67-77. These articles describe systems for identifying and isolating both stuck-faults and intermittent faults in highly complex digital processing systems with confidence levels in the 90-95% range.
A very significant problem associated with such fault tolerant systems is determining their effectiveness in performing the intended tasks of fault identification and isolation. Heretofore, these systems were tested by manually introducing errors into the fault tolerant systems and then analyzing the results produced by the diagnostics to determine if the introduced fault was identified.
One existing method of introducing stuck faults is to simply short circuit a voltage node within the system to ground (stuck-at-0) or to a supply voltage (stuck-at-1). This method works well for simulating short circuits on busses but fails to simulate stuck faults within modules and fails to simulate intermittent faults.
A known method of introducing intermittent faults is to momentarily short circuit a voltage node within the system to ground or a supply voltage. This method has the same limitations as the stuck fault method of injection in that it is impossible to simulate faults at voltage nodes other than those readily accessible. In addition, with the manual injection of intermittent faults, it is virtually impossible to time the injection of the fault with the operation of the system under test. Intermittent faults cannot be placed in time relative to a particular state of a system under test and, therefore, a complete and thorough test is never guaranteed.